InAs/AlSb Quantum Cascade Lasers
R. Teissier, J.C. Moreno, and A. N. Baranov
Institut d’Électronique du Sud
CNRS / Université Montpellier 2
34095 Montpellier, France
InP-based QCL is a dominant technology for mid-IR semiconductor lasers. However,
intrinsic material properties — namely, insufficient conduction band offset — makes difficult the
realization of efficient lasers much below λ=4 µm. We will review here the developments of an
alternative technology based on InAs/AlSb materials that allowed the realization of very short
wavelength QCLs. For eight years, the shortest wavelength QCL emission was about 3.5 µm [1].
The year 2006 has seen remarkable breakthroughs with three different technologies, based on
high conduction band offset materials, that approached the wavelength mark of 3 µm:
InGaAs/AlAsSb lattice matched on InP at the University of Sheffield [2]; strained
(Ga)InAs/Al(In)As heterostructures grown on InP at the Humboldt University of Berlin [3]; and
InAs/AlSb on InAs substrate at the University of Montpellier [4]. More recently, InAs/AlSb
devices push back the short wavelength frontier of QCLs well below 3 µm [5] and exhibited
improved performances in the 3 µm band, offering a new solution for optical gas sensing
applications, particularly at the most wanted wavelength of 3.3 µm corresponding to the
fundamental absorption lines of methane and other hydrocarbons [6].
These QCLs employ a plasmon enhanced waveguide consisting of thick heavily doped InAs
cladding layers separated from the active zone by short period InAs/AlSb superlattices. The
active zone of the devices can be based on different design schemes such as bound-to-continuum
or two phonon depopulation, using vertical or diagonal transitions. The InAs/AlSb
heterostructures are grown by solid source molecular beam epitaxy on n-InAs (100) substrates,
using As and Sb valved cracker cells. The active region of the lasers are usually tellurium-doped
using an Sb2Te3 effusion cell while the n+-InAs cladding layers are doped with silicon. The
lasers are fabricated by conventional micro-processing techniques and using wet chemical
etching.
With the increasing maturity of this technology it is now possible to exploit the attractive
intrinsic properties of the materials InAs/AlSb, which are: a high conduction band offset of 2.1
eV, a large Г-L separation of >0.73 eV and a small electron effective mass of 0.023 m0 in InAs.
A record short wavelength QCL emitting at 2.6 µm has been demonstrated (Figure.1). In
addition to short wavelength operation, these properties must lead to high intrinsic optical gain
and efficient operation at high temperatures. As a matter of facts, lasers operating in the range
3.0 – 4.0 µm, up to temperatures of 400 K have been fabricated (Figure.2). Typical threshold
current densities in the range 2 – 3 kA/cm2 and peak optical powers of the order of 1 W are
measured at room temperature, as illustrated on Figure.3 with a laser emitting at 3.3 µm.
Moreover, single mode DFB lasers with similar performances have been realized, using at first
order surface grating. Continuous wave operation at Peltier cooler temperature has also been
obtained recently. This demonstrates the ability of InAs/AlSb QCL technology to provide
efficient lasers emitting in the 3 µm band and to address applications such as optical gas sensing
in this wavelength range
500
Tmax (K)
400
300
200
100
0
2,5
3,0
3,5
4,0
4,5
5,0
(µm)
Figure.2. Maximal pulsed operation temperature
of today’s InAs/AlSb QCLs as a function of
emission wavelength.
Figure.1. Voltage-current and light-current
characteristics of a InAs/AlSb QCL emitting at a
record wavelength of 2.63 µm.
1,2
D385-22
HR back facet
12 µm x 4 mm
100 ns / 1 kHz
280 K
320 K
300 K
0,6
340 K
0,4
5
360 K
T0=175K
2
Jth (kA/cm )
300 K
0,8
10
D385
12 µm x 4 mm
1 kHz, 100 ns
1,0
Peak Power (W)
Voltage (V)
15
10
as cleaved
300K
HR back facet
1
0,2
3.2 3.4
µm)
380 K
0
400 K
0
1
2
3
Current (A)
4
0,0
100
200
300
400
T (K)
Figure. 3. Voltage-current, light-current characteristics and threshold current density of
InAs/AlSb QCL emitting at 3.3 µm, as a function of operating temperature.
References
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